To manufacture a polymer film used in optical applications, a casting die casts dope onto a casting support to make a cast film. The cast film is peeled from the casting support and is dried to be a film. Then, the film is wound around a winding shaft into a roll. This is a manufacturing method of the film called solution casting method. Cellulose acylate is used as polymer, and triacetyl cellulose (TAC) is one of the cellulose acylate.
Japanese Patent Laid-Open Publication No. 2002-311240 discloses to stretch a TAC film manufactured by solution casting facilities by an off-line stretching line separately built from a solution casting line for the purpose of improving smoothness, mechanical strength, optical properties and the like (hereinafter called off-line stretch).
For the sake of efficient off-line stretch, as disclosed in Japanese Patent Laid-Open Publication No. 2002-311240, it is preferable to continuously stretch a film. Thus, when a plurality of film rolls are subjected to the off-line stretch, a front end of a film fed from a film roll is superimposed on a rear end of another film fed from a preceding film roll, and a superimposed portion is joined together by a double-sided adhesive tape.
Stretching the TAC film in a width direction, however, causes a necking phenomenon, in which the middle of the TAC film in the width direction shrinks largely in a film conveyance direction though both widthwise ends of the TAC film clipped by a tenter hardly shrink. In the superimposed portion, the necking phenomenon occurs in both of the TAC film and the double-sided adhesive tape. The TAC film shrinks more largely than the adhesive tape, so that the adhesive tape often appears from the superimposed portion after stretch. The appearance of the adhesive tape allows an adhesive to adhere to a conveyor roll and the like, resulting in a conveyance problem of frequent occurrence. It is conceivable, on the other hand, that the length of the superimposed portion in the film conveyance direction is made enough larger than that of the adhesive tape to prevent the appearance of the adhesive tape. In this case, however, an unjoined portion of the superimposed portion tends to be caught on the conveyor roll and the like, and brings about another conveyance problem.